1. Field of the Invention
The present invention relates to a method of holographic recording and reproduction, a holographic recording medium and a holographic recording and reproducing apparatus.
2. Description of the Related Art
Holographic storage allows storing data at far higher density compared to planar recording such as conventional magneto-optical recording or phase-change optical recording. Thus, the holographic storage has been proposed as an optical recording system that can achieve high capacity and high-speed transfer, and has been developed actively (see, for example, H. J. Coufal, Holographic Data Storage (Springer, Berlin, 2000), page 3).
Holographic storage is a technique to record information to a recording medium using holography in which an information beam carrying image information is superimposed with a reference beam inside the recording medium and an interference pattern generated in the operation is written to the recording medium. The recorded information is read out by irradiating the recording medium with the reference beam and picking up the image information based on diffraction from the interference pattern.
Because the interference pattern is written three-dimensionally in the holographic storage making use of a thickness of the recording medium, high selectivity in light wavelength and angle is attained, which enables multiplexing, and therefore, high recording capacity can be achieved. In addition, since the image information can be read out at once, holographic storage provides an advantage of high transfer speed of reconstruction signals.
The image information to be recorded is generally generated by encoding digital information. In reproduction, the recorded image information is read out, decoded, and extracted as information.
In recent years, reflection collinear holographic storage has been proposed as a novel recording system with remarkably increased practicability compared to conventional transmission holographic storage (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-123949). This system uses a recording medium having a recording layer on the front side and a reflection layer on the rear surface in which recording is performed by irradiating the recording layer with a reference beam and an information beam on a coaxial optical path. Since this system allows performing input and output of information in the same direction and on the same axis with respect to the medium, a pickup can be easily mechanical-controlled. In addition, because address information of the recording medium can be read out under tracking and servo based on information pre-formatted on the medium, random access is enabled. Furthermore, since recording and reproduction are performed by irradiating the medium with spherical waves focused by a lens, reproduction can be advantageously performed even if the medium in a disk is tilted.
In the above-mentioned holographic storage of the transmission system, reflection collinear system, or advanced collinear system, it is assumed that a laser diode is used as a light source to record and reproduce the hologram. Because of a small size and low power consumption, the laser diode has already been used in recording and reproduction apparatuses for various optical recording media such as CD-ROM and DVD, and is also a promising candidate for a light source for holographic recording and reproduction apparatuses. However, the laser diode has problems that must be solved when used in the holographic recording and reproduction.
It is known that laser diodes have individual difference in emission wavelength. The individual difference means that, even if the lasers are designed to emit light of a specific wavelength, emission wavelengths thereof may vary from several nanometers to ten nanometers depending on individual lasers due to slight variations in material compositions, thicknesses of respective layers, or the like. In order to manufacture laser diode free of wavelength variation, manufacturing conditions must be stringently controlled, resulting in poor yield and increased manufacturing cost.
In reproducing information recorded as the hologram, it is necessary to read the information using a reproducing beam of the same wavelength as that of the recording beam. However, under the circumstances that the laser diode may have a wavelength variation, there may be a case where reproduction cannot be successfully achieved when the hologram is to be reproduced with an apparatus other than that apparatus used in recording. Specifically, the wavelength variation in the reproducing beam brings about lowering in the signal-to-noise ratio because reconstruction beam intensity is reduced due to decrease in diffraction efficiency of the reproducing beam or an outgoing direction of the beam is varied from a designed direction.
The emission wavelength of a laser diode may vary in accordance with an operating temperature or an injection current, and also may vary depending on an elapsed time. In the case where the volume of recording medium is changed, a wavelength suitable to reproduction may differ from that used in recording. Under the circumstances, the SNR may also be lowered so that the reproduction is made difficult even if reproduction is to be performed with the apparatus same as that used in recording.
To these problems, in the transmission holographic storage system, it is proposed to read positional information on the optical detector array of the reconstruction signal and to control the light source wavelength (Jpn. Pat. Appln. KOKAI Publication No. 2002-216359). However, in the reflection collinear or advanced collinear holographic storage system, reproduction becomes markedly difficult as compared to the transmission holographic storage system unless the wavelength variation is controlled more strictly. Therefore, a method capable of detecting the wavelength variation between the recording beam and the reproducing beam more easily is required.